Artificial agglomerate stone article comprising synthetic silicate granules

ABSTRACT

The invention relates synthetic silicate granules comprising a mixture of SiO 2 , Al 2 O 3  and Na 2 O, which can be obtained by sintering; to their use in manufacturing an agglomerate stone material and to the agglomerate stone material resulting thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/EP2020/071521, filed on Jul. 30, 2020,which claims priority to European Patent Application No. 19382661.7,filed on Jul. 31, 2019, the entire contents of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The invention is related to materials for construction, decoration andarchitecture, made of artificial agglomerate stone, as well as to theirmanufacture and fabrication. Particularly, the invention falls withinthe technological field of artificial stone articles composed ofinorganic fillers selected from stone, stone-like or ceramic materials,and a hardened organic resin, manufactured by a process which includesvacuum vibrocompaction and hardening of unhardened agglomerate mixtures.

BACKGROUND OF THE INVENTION

Artificial agglomerate stone articles which simulate natural stones,also known as engineered stone articles, are common in the construction,decoration, architecture and design sectors. The processes for theirmanufacture at industrial scale are well established nowadays.

One of most popular artificial stone materials, highly appreciated bytheir aesthetic, hardness and resistance to staining and wear, are theso-called quartz agglomerate surfaces. They are extensively used forcountertops, claddings, floorings, sinks and shower trays, to name a fewapplications. They are generally called artificial stones, and theirapplications coincide with the applications of stones such as marble orgranite. They can be made simulating the colors and patterns in naturalstone, or they might also have a totally artificial appearance, e.g.with bright red or fuchsia colors. The basis of their composition andthe technology currently used for their manufacture dates back from thelate 1970s, as developed by the Italian company Breton SpA, and which isnowadays commercially known in the sector under the name Bretonstone®.The general concepts hereof are described, for example, in the patentpublication U.S. Pat. No. 4,204,820. In this production process, quartzand/or cristobalite stone granulate, having varied particle sizes, arefirstly mixed with a hardenable binder, normally a liquid organic resin.The resulting mixture is homogenized and distributed on a temporarymold, wherein it is then compacted by vibrocompaction under vacuum andsubsequently hardened.

A different sort of artificial agglomerate materials is the generallyknown ‘solid surface’. With this rather indefinite term, the industryrefers to construction materials of hardened (mostly acrylic) organicresin with ATH (alumina trihydrate, bauxite) as predominant filler. Suchproducts are produced by cast-molding the liquid acrylic resin and ATHflowable mixture, optionally together with vibration to remove airbubbles, and then heat hardening the mixture. Due to the requirement ofenough flowability to facilitate casting and air removal, the amount ofliquid resin is normally not lower than 20 wt. % of the uncured mixture.In comparison with quartz surfaces, solid surfaces suffer from lowerhardness and wear resistance, and are inferior when trying to mimic theappearance of natural stones (the user associates them with plasticcomposites, and not with natural stones).

Other combinations of stone granulate filler and binder have beenproposed, with varied commercial success. Thus, for example, marble andgranite have been tried as granulates for agglomerates together withorganic resins, but they resulted in materials with significantly lowerperformance than quartz surfaces for their use as construction materialsand highly limited possibilities regarding their appearance. Myriad ofother mineral and non-mineral granulate fillers have been described,mostly in the patent literature, such as recycled glass, glass frits,glass beads, feldspars, porphyry, amorphous silica, ceramics, dolomite,basalt, carbonates, metal silicon, fly-ash, shells, corundum, siliconcarbide, among many others. On the other hand, inorganic binders, suchas hydraulic cement, have been used instead of organic resins incommercial agglomerate artificial stone for building applications.

Quartz and cristobalite are two of the most common crystalline forms ofsilica (SiO₂) in nature, cristobalite being significantly less frequent.Quartz is present in all types of rocks, igneous, metamorphic andsedimentary. Cristobalite is a high temperature crystalline polymorph ofsilica, formed in nature as result of volcanic activity, orartificially, by the catalyzed conversion of quartz at high temperaturein a rotary kiln. Both quartz and cristobalite have high melting points,high hardness, they are translucent or transparent, and relatively inertto chemical attacks. These properties, together with their abundance andavailability, have made them extremely useful as granulate filler forquartz surfaces. Cristobalite is furthermore used in those materials dueto its outstanding whiteness. The amount of quartz/cristobalite in thosematerials normally range from 50-95 wt. %, the rest being otherinorganic fillers and the hardened organic resin.

As mentioned above, quartz and cristobalite have several characteristicsthat make them ideal fillers for the application in the manufacture ofdurable construction/decoration surfaces, such as high abundance andavailability, hardness, translucency, whiteness and chemical inertness.However, they have at least one very serious drawback. The fine fractionof respirable crystalline silica dust generated during the manufactureof the artificial agglomerate stone containing quartz or cristobalite,or when this agglomerate material is mechanically processed, possess aserious occupational health risk for workers or fabricators. Prolongedor repeated inhalation of the small particle size fraction ofcrystalline silica dust has been associated with pneumoconiosis(silicosis), lung cancer and other serious diseases. To avoid thishazard, workers potentially exposed to high levels of the respirablefraction of crystalline silica dust are required to wear personalprotection equipment (e.g. respirators with particle filter), to workunder ventilation for efficient air renewal and to use measures whichfight the source of the dust (e.g. processing tools with water supply ordust extraction).

To cope with this shortcoming from the raw material side, naturalmaterials such as feldspar could be proposed as substitute of quartzand/or cristobalite in quartz surfaces. Indeed, feldspar has beendescribed as suitable filler in this type of products, for example inEP2011632A2 examples 1 or 2. However, the problem with natural rawmaterials is the variability in their characteristics, such as color,composition, transparency, etc. Feldspar and other natural minerals arefurthermore very frequently accompanied by substantive amounts ofquartz.

Ceramics have been sometimes mentioned as possible fillers in artificialstone agglomerate, as in EP 2409959 A1, although without giving anyparticulars of the type of ceramic material or their advantages. Glassparticles and glass beads have often been described as suitableinorganic particulate fillers, for example in EP 1638759 A1. Althoughglass particles have some characteristics interesting for their use asfillers, such as their transparency or the absence of crystalline silicain their structure, their comparatively excessive production cost haslimited their use. The replacement of new glass by glass cullet (glassparticles recovered from industrial or urban glass waste), as describedfor example in U.S. Pat. No. 5,364,672 A, has not been a satisfactorymore economical alternative, due to its variability and the nearlyunavoidable presence of cumbersome contaminants in this recycledmaterial.

It has been suggested in the past the use of glass frits as inorganicgranulate filler, for example in WO2018189663A1. Although this referencedoes not sufficiently describe how the frits can be produced, thesematerials are normally made from quartz as main raw materials (as otherglasses), which needs to be fused totally to reduce the crystallinesilica content. This exhaustive amorphization requires high temperaturesaround 1.400° C.-1.700° C. and long furnace residence times (severalhours), which goes together with high energetic costs. Apart from thisdifficult manufacture and excessive cost, from the disclosure inWO2018189663A1 it is not clear whether the properties and the visualappearance of the frit granulate comes close to the properties andappearance of quartz or cristobalite granules, and whether thepotentially obtainable agglomerate products, would comply with the highaesthetic and mechanical demands for this type of agglomerate material.Furthermore, in the production of frit materials, the molten glassstream of sufficiently low viscosity is rapidly quenched with cold waterto maintain the vitreous molecular structure (to avoidrecrystallization) and ground. Before they can be used, the fritgranules need to be dried at sufficiently high temperature to removemost of the water, which requires additional energy input, and finallygrinded to the desired particle size.

From this background it is obvious that there is still a need for analternative synthetic material obtainable in granule form at affordablecost, for its use as filler in artificial agglomerate stone articleswhich has a combination of the following advantages:

-   -   It can be produced from readily available raw materials and at a        competitive cost;    -   It does not generate troubling levels of respirable crystalline        silica during handling or processing,    -   It does not limit the chromatic effects and color richness of        the currently available quartz agglomerate articles;    -   It can be used with minor modifications in the currently        available industrial manufacturing processes for quartz        agglomerate articles; and    -   It does not impair the performance of the agglomerate articles        when compared to current quartz agglomerate articles, in terms        of scratch resistance, durability, stain- and        chemical-resistance.

SUMMARY OF THE INVENTION

The invention is based on the finding by the inventors, after extensiveresearch and experimentation, that certain types of synthetic silicategranules, which can be defined by their chemical composition of specificmetal oxides, can be used as fillers of excellent whiteness in themanufacture of artificial agglomerate stone articles or materialsreplacing quartz and/or cristobalite granules. These synthetic silicategranules do not suffer from the shortcomings observed for the fillersalternative to quartz and/or cristobalite described previously.

Thus, in a first aspect, the invention is concerned with syntheticsilicate granules comprising:

-   -   52.50-59.80 wt. % of SiO₂,    -   33.50-41.10 wt. % of Al₂O₃, and    -   0.30-3.10 wt. % of Na₂O,

based on the weight of the synthetic silicate granules.

In a second aspect, the invention is concerned with the use of syntheticsilicate granules as defined in the first aspect, for the manufacture ofan artificial agglomerate stone material.

A third aspect of the invention refers to an artificial agglomeratestone material comprising inorganic fillers and a hardened binder,wherein the inorganic fillers comprise the synthetic silicate granulesas defined in the first aspect.

In a fourth aspect, the invention is concerned with the use of syntheticsilicate granules as defined in the first aspect in an artificialagglomerate stone material, to reduce the emissions of crystallinesilica when the material is manufactured and/or mechanized (i.e. cut,gauged, polished, etc.).

In a fifth aspect, the invention is directed to a process for preparingan artificial agglomerate stone material as defined in the third aspect,comprising:

-   -   a) mixing a hardenable binder and the inorganic fillers        comprising the synthetic silicate granules as defined in the        first aspect,    -   b) vacuum vibrocompacting the mixture obtained in a), and    -   c) hardening the compacted mixture obtained in b).

In a sixth aspect, the invention is directed to a process of the fifthaspect, wherein the synthetic silicate granules are obtained bysintering a mixture of kaolin and a flux, the flux being preferablyselected from feldspar, calcite and dolomite, or mixtures thereof, andwherein the weight ratio of kaolin to flux is preferably from 95:5 to75:25.

DETAILED DESCRIPTION OF THE INVENTION

The synthetic silicate granules according to the different aspects ofthe invention have the characteristics of having an excellent whiteness,some level of transparency, and when mixed with resin, they do notpresent an important color deviation from the color of high-qualityquartz and/or cristobalite. The granules furthermore present goodhomogeneity, high hardness, good resistance to chemical attack, lowporosity, low level of defects and low content of crystalline silica.Furthermore, since the sintering temperatures are lower than the typicalglass fusion temperatures, and since no water drying step is required,the synthetic silicate granules can be produced at a lower energeticcost than the glass or frit alternatives.

In the present application, the term “granules” usually refers toindividual units (particles). Thus, the term encompasses units rangingfrom infinitesimal powder particulates with sizes on the micrometerscale up to comparatively large pellets of material with sizes on themillimeter scale. This term encompasses particulate products of avariety of shapes and sizes, including grain particles, fines, powders,or combinations of these.

Also, in the present application the term “synthetic” is used toindicate that the material is obtained by man-made transformation of rawmaterials, e.g. by thermal or chemical processes, into a mass of adifferent substance, normally not present as such in nature, and whichcannot be separated back to the starting raw materials. In particular,the synthetic silicate granules of the present invention are preferablyobtained by thermal treatment of selected raw materials, and morepreferably, the synthetic silicate granules are ceramic granules.

The particle size, also called particle diameter, of the granules can bemeasured by known screening separation using sieves of different meshsize. The term “particle size” as used herein, means the range in whichthe diameter of the individual particles in the synthetic silicategranules falls. It can be measured by particle retention or passage oncalibrated sieves that have measured mesh size openings, where aparticle will either pass through (and therefore be smaller than) or beretained by (and therefore larger than) a certain sieve whose sizeopenings are measured and known. Particle sizes are defined to be withina certain size range determined by a particle's ability to pass throughone sieve with larger mesh openings or ‘holes” and not pass through asecond sieve with smaller mesh openings. For synthetic silicate granuleswith a particle size <200 micrometers, the particle size distribution ofa granule sample can be measured by laser diffraction with a commercialequipment (e.g. Malvern Panalytical Mastersizer 3000 provided with aHydro cell). For the measurement, the granule sample might be dispersedin demineralized water assisted by an ultrasound probe. The laserdiffractomer provides particle distribution curves (volume of particlesvs. particle size) and the D10, D50 and D90 statistical values of theparticle population of the sample (particle size values where 10%, 50%or 90% of the sample particle population lies below this value,respectively).

The composition of the granules might be obtained by X-ray fluorescence(XRF), a technique well-established in the mineral technological field.The composition of the granules indicated herein corresponds preferablyto the average, calculated from at least 3 repetitions of themeasurement, of the composition of samples containing a mass of granules(e.g. 1 gram of granules).

The skilled person readily understands that, when a composition ormaterial is defined by the weight percentage values of the components itcomprises, these values can never sum up to a value which is greaterthan 100%. The amount of all components that said material orcomposition comprises adds up to 100% of the weight of the compositionor material.

The synthetic silicate granules of the different aspects of theinvention are characterized by a composition which comprises oxidesaccording to the following ranges in weight percent, based on the weightof the synthetic silicate granules:

Range (wt. %) SiO₂ 52.50-59.80 Al₂O₃ 33.50-41.10 Na₂O 0.30-3.10

It needs to be understood that the synthetic silicate granules have acombination of the composition ranges in the preceding table.

Preferably, the synthetic silicate granules comprise also 56.90-59.80 wt% of SiO₂ based on the weight of the synthetic silicate granules.

The synthetic silicate granules comprise preferably also 33.50-40.10 wt.% of Al₂O₃ based on the weight of the synthetic silicate granules.

The synthetic silicate granules comprise preferably also 0.90-3.10 wt. %of Na₂O based on the weight of the synthetic silicate granules.

In a preferred embodiment, the synthetic silicate granules arecharacterized by a composition which comprises oxides according to thefollowing ranges in weight percent, based on the weight of the granules:

Range (wt. %) SiO₂ 56.90-59.80 Al₂O₃ 33.50-40.10 Na₂O 0.90-3.10

There might be other inorganic oxides present in the composition of thesynthetic silicate granules, as well as some organic matter or materialwhich is calcined and desorbed during the XRF analysis at 1050° C. untilthere is no more weight lost (known as weight ‘lost on ignition’ orL.O.I.).

Nevertheless, the sum of the weight percentages of the SiO₂, Al₂O₃ andNa₂O in the granules is preferably at least 90 wt. %, or at least 95 wt.%, based on the weight of the granules. The sum of the weightpercentages of the SiO₂, Al₂O₃ and Na₂O in the granules might be in therange 86.30-99.80 wt. %, preferably 90.00-99.50 wt. %, or 95.00-99.50wt. %, based on the weight of the granules. Preferably, the rest beingother inorganic oxides and other matter lost on ignition (L.O.I.).

Also, preferably, the L.O.I. is lower than 4.00 wt. %, more preferablylower than 1.00 wt. %, or lower than 0.50 wt. %, based on the weight ofthe granules. In a further embodiment, the amount of L.O.I. is in therange 0.01-1.00 wt. %, or 0.01-0.50 wt. %, based on the weight of thegranules.

The synthetic silicate granules may further comprise CaO in thecomposition, preferably in a range 0.10-6.90 wt. %, or 0.10-4.00 wt. %,or 0.10-2.00 wt. %, based on the weight of the granules.

The synthetic silicate granules may further comprise MgO in thecomposition, preferably in a range 0.10-3.10 wt. %, or 0.10-2.00 wt. %,or 0.10-1.00 wt. %, based on the weight of the granules.

The synthetic silicate granules might further comprise K₂O in a range0.00-2.00 wt. %, or 0.10-1.00 wt. % relative to the weight of thegranules.

Iron oxides, and particularly Fe₂O₃, might be present in the compositionof the granules, however, preferably, the average concentration of Fe₂O₃is 1.00 wt. %, or more preferably 0.60 wt. %, based on the weight of thegranules. In an embodiment, iron oxides, and particularly Fe₂O₃, mightbe present in the composition of the granules in a concentration of0.00-1.00 wt. %, or more preferably 0.00-0.60 wt. %, based on the weightof the granules. In a further embodiment, iron oxides, and particularlyFe₂O₃, might be present in the composition of the granules in aconcentration of 0.10-1.00 wt. %, or more preferably 0.10-0.60 wt. %,based on the weight of the granules.

Titanium dioxide TiO₂ might also be present in the composition of thegranules. In that case, the average concentration of TiO₂ in thegranules is 0.50 wt. %, preferably 0.30 wt. %, based on the weight ofthe granules. In an embodiment, TiO₂ might be present in the compositionof the granules in a concentration of 0.00-0.50 wt. %, or morepreferably 0.00-0.30 wt. %, based on the weight of the granules. In afurther embodiment, TiO₂ might be present in the composition of thegranules in a concentration of 0.10-0.50 wt. %, or more preferably0.10-0.30 wt. %, based on the weight of the granules.

The concentration of both Fe₂O₃ and/or TiO₂ can be adjusted to this lowranges by selection of raw materials with particularly low levels ofthose oxides.

Further, in preferred embodiments, the water content of the syntheticsilicate granules is preferably <0.50 wt. %, more preferably <0.10 wt.%, based on the weight of the granules. It has been found that if watercontent is higher, the hardening of the binder, e.g. the curing of theresin, and the adhesion of the granules to the binder, might bedetrimentally affected. As an additional advantage of the syntheticsilicate granules of the invention in comparison with glass frits, thefirst granules do not require a drying step to achieve the mentionedlevel of water content, while glass frits do (glass frits are oftenproduced by pouring the molten glass to cold water).

Therefore, in a preferred embodiment, the synthetic silicate granulesmay comprise 0.00-0.50 wt. % of water, more preferably 0.00-0.10 wt. %,based on the weight of the granules. In a further embodiment, thesynthetic silicate granules may comprise 0.01-0.50 wt. % of water, morepreferably 0.01-0.10 wt. %, based on the weight of the granules.

According to an embodiment, the synthetic silicate granules of thedifferent aspects of the invention are characterized by a compositionwhich comprises oxides according to the following ranges in weightpercent, based on the weight of the synthetic silicate granules:

Range (wt. %) SiO₂ 52.50-59.80 Al₂O₃ 33.50-41.10 Na₂O 0.30-3.10 CaO0.10-6.90 MgO 0.10-3.10 K₂O 0.00-2.00 Fe₂O₃ 0.00-1.00 TiO₂ 0.00-0.50

In a further embodiment, the synthetic silicate granules comprise oxidesaccording to the following ranges in weight percent, based on the weightof the synthetic silicate granules:

Range (wt. %) SiO₂ 56.90-59.80 Al₂O₃ 33.50-40.10 Na₂O 0.90-3.10 CaO0.10-4.00 MgO 0.10-2.00 K₂O 0.10-1.00 Fe₂O₃ 0.00-0.60 TiO₂ 0.00-0.30

The synthetic silicate granules may comprise silica in crystalline form(as quartz or cristobalite). However, preferably, the crystalline silicaconcentration in the granules is 15 wt. %, or 10 wt. %, or even 8 wt. %,based on the weight of the granules. In an embodiment, the crystallinesilica concentration in the granules is in the range 0-15 wt. %, or 0-10wt. %, or even 0-8 wt. %, based on the weight of the granules. In anembodiment, the crystalline silica concentration in the granules is inthe range 0.1-15 wt. %, or 0.5-10 wt. %, or even 1.0-8 wt. %, based onthe weight of the granules. Preferably, the crystalline silicaconcentration in the granules is in the range 1.0-15.0 wt. %, or 3.0-15wt. %, or even 3.0-10 wt. %, based on the weight of the granules. Thelow crystalline silica content in the synthetic silicate granules is aconsequence of the low crystalline content of the raw materials used fortheir production, and of the partial vitrification during thermaltreatment.

The synthetic silicate granules are preferably not frits, meaning thatthey are not produced by fusing/melting fully a glass composition whichis rapidly cooled (quenched).

The synthetic silicate granules are preferably made of sinteredmaterial, meaning that they are obtained by a sintering process ofinorganic raw materials. In other words, the synthetic silicate granulesare preferably not fully, of substantially not fully, amorphous, and thecrystalline phase in the granules is preferably >1 wt. %, or in therange 5-80 wt. %, in relation to the weight of the granules. Thesynthetic silicate granules are preferably ceramic granules.

The term “ceramic granules” refers to granules consisting of inorganic,non-metallic compounds, that are consolidated in solid state by means ofhigh temperature heat treatments (firing, sintering) and are formed by acombination of crystalline and glassy phases.

According to some embodiments, the amount of the crystalline phasemullite (Al₆Si₂O₁₃) accounts for 20-60 wt. %, or 30-50 wt. % of theweight of the synthetic silicate granules. In preferred embodiments ofthe invention, the amount of the crystalline phase mullite (Al₆Si₂O₁₃)accounts for 15-60 wt. %, or 20-50 wt. % of the weight of the syntheticsilicate granules. The amount of crystalline silica and mullite in thesynthetic silicate granules can be determined by powder X-RayDiffraction analysis (XRD) using the Rietveld method for quantification,a technique amply used in the field.

The total content of crystalline phases in the synthetic silicategranules according to any aspect of the invention is preferably 5 wt. %,or 10 wt. % or even 20 wt. % of the weight of the granules, and alsopreferably <30 wt. %, or 70 wt. % of the weight of the granules, therest being amorphous phase. In preferred embodiments of the invention,the amount of the crystalline phases in the synthetic silicate granulesaccording to any aspect of the invention is preferably 5-80 wt. %, or10-80 wt. %, 20-80 wt. %, or even 20-70 wt. % of the weight of thegranules. In an embodiment, the crystalline phase mullite accounts for20-60 wt. %, or 30-50 wt. % of the weight of the synthetic silicategranules. Preferably, the crystalline phase mullite accounts for 15-60wt. %, or 20-50 wt. % of the weight of the synthetic silicate granules.

Preferably, the synthetic silicate granules according to the aspects ofthe invention might have a particle size in a range from 2.0-0.063 mm(grain particles) or it might be lower than 63 micrometers (micronizedpowder). In the case of grain particles, the particle size might rangefrom 1.2-0.1 mm, or 0.7-0.3 mm, or 0.4-0.1 mm, or 0.3-0.063 mm. In thecase of micronized powder, the powder might have a particle sizedistribution with a D90<50 micrometers, preferably <40 micrometers, andmore preferably the D90 might be between 10-40 micrometers. Optionally,different fractions of synthetic silicate granules, with differentparticle size distribution, may be included in the artificialagglomerate article of the invention.

In any aspect of the invention, it is particularly preferred when thesynthetic silicate granules comprised in the artificial agglomeratestone article have a particle size 0.4 mm. In addition, in preferredembodiments the amount of synthetic silicate granules as micronizedpowder with a particle size 0.063 mm is 10-40 wt. % in relation to theweight of the artificial agglomerate stone material.

Synthetic silicate granules according to the present invention can beprepared by a process comprising:

-   -   (a) preparing a mixture comprising kaolin and a flux, preferably        wherein the weight ratio of kaolin to the flux is from 95:5 to        75:25;    -   (b) compacting the mixture of step (a); and    -   (c) sintering the compacted mixture of step (b).

The term flux is used with its generally accepted meaning, i.e. meaninga substance of an inorganic oxide that lowers the melting, sintering orsoftening temperature of the mixture with kaolin. The flux is preferablyselected from feldspar, calcite, dolomite, and mixtures thereof.

As used herein, sintering shall be understood as the process ofsubjecting an inorganic mixture to a thermal treatment (normally over900° C.) to form a solid mass from the starting materials, by theirpartially fusion and reaction, but without reaching the point of fullliquefaction.

The sintering might be conducted in a furnace at temperatures of900-1.450° C., preferably 900-1.300° C. Preferably, the sinteringtemperature is not higher than 1.450° C.

The weight ratio of kaolin:flux is preferably selected in the range95:5-75:25. That is, the following formula applies:

$3 < \frac{{Weight}{}{kaolin}}{{Weight}{flux}} < 19$

The kaolin is preferably white kaolin, a low iron kaolin of high purity,which is a natural clay mined and available from different suppliers,for example Imerys, Sibelco, among others. The kaolin comprisespreferably >80 wt. % of kaolinite (Al₂Si₂O₅(OH)₄), with >30 wt. % ofAl₂O₃ content and 0-0.1 wt. % Fe₂O₃, based on the weight of the kaolin.

Kaolin refers to a clay containing the mineral kaolinite as itsprincipal constituent. Preferably, kaolinite is the only plasticcomponent in kaolin. Kaolin may further contain other impurities, suchas quartz, mica, phosphates, fine clay impurities such as certainsmectite clay constituents and various other species, e.g. compoundscontaining transition elements. In a particular embodiment, kaolincomprises at least 80 wt % of kaolinite, based on the weight of kaolin.

The flux might be selected from feldspar, calcite, dolomite, and/ormixtures thereof.

The flux is preferably feldspar. Felspars are aluminosilicatescontaining sodium, potassium, calcium or barium. More preferably, thefeldspar is sodium feldspar (albite). The felspar is preferably a lowiron sodium feldspar of high purity with >10 wt. % NaO and 0-0.1 wt. %Fe₂O₃, with low quartz content, preferably of 0.1-10.0 wt. %, based onthe weight of the feldspar material. This type of feldspar is extractedfrom mines and commercialized by companies such as Sibelco or Imerys.

The flux may be calcite. Calcite (calcium carbonate) can be used assuch, or it can be used in calcined form (calcium oxide, quicklime).Both calcium carbonate and calcium oxide might be interchangeably orsimultaneously used as a source of calcium.

Preferably, calcite as such is preferred, in the form of high puritycalcite (mineral composed primarily of CaCO₃) with 50-56 wt. % of CaOand <0.1 wt. % of Fe₂O₃. Dolomite refers preferably to dolomite (mineralformed mainly by CaMg(CO₃)₂) with 18-48 wt. % of MgO and <0.1% Fe₂O₃.

Optionally, the flux might comprise a mixture of sodium feldspar andcalcite as described above. The amount of calcite might be in a range of1.0-50.0 wt. %, or 5.0-40.0 wt. %, based on the weight of the flux,while the amount of feldspar might range from 50.0-99.0 wt. %, or 60-95wt. %, based on the weight of the flux.

Preferably, the synthetic silicate granules are produced by a methodthat does not involve any step in which the temperature is increasedabove 1.450° C. for more than 5 minutes, or for any extension of time.

The mixture is preferably introduced in the sintering furnace ingranular form, as spheres, grains, pellets, briquettes or the like, witha maximum size in any dimension of ≤10 mm, preferably ≤5 mm, and evenmore preferred ≤4.5 mm. The minimum size of the granular form in anydimension is preferably ≥0.045 mm, more preferably ≥0.060 mm.

Preferentially, both the kaolin and the flux (e.g. feldspar, calcite,dolomite) are previously ground and selected to have a particle size of<150 micrometers, or preferably <100 micrometers, and preferably >1micrometer, before they are mixed and compacted. The small raw materialparticle size translates into a more homogeneous mixing and moreefficient sintering, what means that less energy is necessary to producethe sintering of the mixture and the synthetic silicate granulesobtained present less defects, inclusions or inhomogeneities.

The kaolin and the flux are preferably mixed, homogenized and compactedbefore they are introduced into a sintering furnace.

The mixture of kaolin and the flux, including optional additives, can becompacted by different techniques known in the art. For example, thecompaction can be achieved with an axial press or continuous belt press,by extrusion or by a granulator.

Optionally, known agglomerating additives might be added to the mixtureto be sintered, such as carboxymethylcellulose (CMC), water, bentoniteand/or polyvinylalcohol, which can be added to facilitate the mixtureand the subsequent compaction. Agglomeration additives are preferablyused in small amounts, preferably 0-5 wt. %, based on the weight of themixture to be sintered.

In preferred embodiments, the kaolin and the flux accounts for more than85 wt. %, or >90 wt. %, or >95 wt. % of the mixture to be sintered.

Preferably, the mixture to be sintered, comprising kaolin and the fluxand the optional components, is granulated by a ceramic granulator (asthose used in the ceramic industry for granulating clay mixtures), torounded or spherical particles before they are introduced into thesintering furnace.

The density of the granules to be introduced into the sintering furnacepreferably ranges 1.0-1.5 g/cm³. The limited granular size of themixture favors heat transfer into the bulk of the mixture andfacilitates a more homogeneous and efficient sintering, reducing therequired temperature and the time of residence of the mixture in thefurnace. A further advantage of the granular form of the compactedmixture is that the size and shape before sintering can be chosen inrelation to the size and shape of the desired sintered syntheticsilicate granules to be used in the artificial agglomerate stone.

The mixture to be sintered (comprising kaolin and flux and the optionalcomponents), preferably in granular form, is introduced into a heatedfurnace to achieve its calcination, sintering and ultimately thetransformation of the raw materials into a single mass of mixedcrystalline and amorphous character. The thermal treatment is preferablyconducted at a temperature <1.450° C., or in other words, preferably themanufacture of the synthetic silicate granules does not involve any stepin which the temperature is increased above 1.450° C. for more than 5minutes, or for any extension of time. Depending on the size and shapeof the compacted mixture introduced into the furnace, the sinteringtemperatures may range from 900-1.450° C., preferably for 5-60 minutes,more preferably for 5-30 minutes. In comparison with the manufacture ofglass ceramics, where full melting of the materials is required, thesynthetic silicate granulates can be produced at a lower temperatureand/or reduced furnace residence time, what is economicallysignificantly advantageous. Further, at high temperatures such as thoseabove 1450° C., cristobalite might start to crystalize from the SiO₂present in the mixture, increasing the total crystalline silica content.

Furnaces for the sintering of the mixture can be any of those used inthe art for firing or calcinating ceramic materials, such as rotary ortunnel kilns, conveyor furnaces, fluidized bed furnaces, furnaces forfiring ceramic beads, vertical or bottom-up furnaces, etc. The furnacescan be designed for batch or continuous operation. Preferably, thesintering is produced in a rotary kiln furnace with continuousoperation.

After the thermal treatment, the sintered product is ground and/orclassified according to the desired particle size distribution(granulometry). The grinding and/or classification (sieving) can beachieved by methods currently known in the art, such as ball mineralgrinding mills or opposed grinding rollers. The grinding may alsocomprise micronizing the sample to obtain granules with a particle size<65 micrometers, or to a powder with a particle size distribution havinga D90<50 micrometers.

In an aspect, the invention refers to the synthetic silicate granulesobtained by the process disclosed herein.

The inventors made the unprecedented observation that the syntheticsilicate granules according to the invention, characterized by theclaimed composition, present an excellent whiteness, moderate level oftransparency, and little color deviation from the color of quartzgranules or cristobalite granules commonly used in the manufacture ofquartz surfaces. The synthetic silicate granules are furthermore hardand with good resistance to chemical attack. It is also observed thatwhen mixed with the unhardened binder, the amount of liquid binderabsorbed by the granules is comparable or lower to the amount absorbedby quartz or cristobalite granules. This feature is particularlyrelevant for the small particle sizes, for the micronized granules. Itneeds to be understood that the low absorption of liquid binder of thismicronized fraction is an advantage in the manufacture of artificialagglomerate articles, since high amounts of absorbed unhardened binderrequires the use of higher amounts of this binder, which is moreexpensive, in order to achieve the same cohesion and granule anchorage.The crystalline silica content of the synthetic silicate granules isvery low, of 15 wt. % or lower, reducing drastically the health riskscaused by inhalation of respirable crystalline silica. This combinationof features allows the replacement of at least part of the quartz and/orcristobalite currently used in the manufacture of quartz surfaces,without having to modify importantly the current formulations and/ormanufacturing processes, and without deteriorating the performance andthe visual appearance of these products. The use of the syntheticsilicate granules instead of quartz and/or cristobalite in artificialagglomerate articles reduces the crystalline silica emissions producedwhen these articles are mechanized.

Therefore, in another aspect, the invention is directed to the use ofthe synthetic silicate granules of the invention for the manufacture ofan artificial agglomerate stone material or article. This use reducesthe crystalline silica emissions during manufacturing or mechanizing theartificial agglomerate stone material or article, compared toagglomerate quartz material or articles.

Accordingly, in a particular embodiment, the invention is directed tothe use of the synthetic silicate granules of the invention for themanufacture of an artificial agglomerate stone material or article, toreduce the emissions of crystalline silica when the material ismanufactured and/or mechanized.

Other aspect of the invention refers to an artificial agglomerate stonematerial or article comprising inorganic fillers and a hardened binder,wherein the inorganic fillers comprise the synthetic silicate granulesof the invention.

The amount of synthetic silicate granules in the artificial agglomeratestone material preferably ranges from 1-70 wt. %, or from 1-50 wt. %, orfrom 1-30 wt. % in relation to the weight of the material.

The artificial agglomerate stone material might comprise also inorganicfillers, e.g. granules, different from the synthetic silicate granulesof the invention, preferably selected from stone, stone-like or ceramicmaterials. Preferably, the inorganic fillers (i.e. the sum of theweights of the synthetic silicate granules and of the inorganic fillersdifferent from the synthetic silicate granules of the invention) accountfor at least 70 wt. %, or at least 80 wt. %, or at least 85 wt. %, andat most 95 wt. %, of the weight of the artificial agglomerate stonematerial.

In equally preferred embodiments, in addition to the synthetic silicategranules according to the invention, the artificial agglomerate stonematerial further comprises other inorganic fillers selected fromfeldspar granules, recycled silicate glass granules, silicate fritgranules, ceramic granules, or mixtures thereof.

The synthetic silicate granules comprised in the artificial agglomeratestone article have preferably a particle size 0.4 mm. In addition, inpreferred embodiments the amount of synthetic silicate granules asmicronized powder with a particle size 0.063 mm is 10-40 wt. % inrelation to the weight of the artificial agglomerate stone material.

The hardenable binder is preferably an organic thermosetting resin,liquid and which may be selected from the group made up of unsaturatedpolyester resins, methacrylate-based resins, vinyl resins and epoxyresins. These hardenable organic resins are preferably reactive and canbe hardened in a curing (or cross-linking) reaction.

The hardening of the binder, and thus, of the mixture after compaction,can ultimately be accelerated by raising the temperature, depending onthe binder used, and/or by using suitable catalysts and accelerators.

The amount of hardened binder in the artificial agglomerate stonematerial may range from 5-30 wt. %, or from 5-20 wt. %, or from 5-15 wt.%, based on the weight of the material.

In an embodiment, the artificial agglomerate stone material comprises70-95 wt. %, preferably 80-95 wt. %, of inorganic fillers (i.e. the sumof the weights of the synthetic silicate granules and of the inorganicfillers different from the synthetic silicate granules of the invention)and 5-30 wt. %, preferably 5-20 wt. %, of hardened binder, based on theweight of the artificial agglomerate stone material.

According to preferred embodiments, the artificial agglomerate stonearticle has been obtained by vacuum vibrocompaction and has preferablyan apparent density in the range 2000-2600 kg/m³, or from 2100-2500kg/m³. Apparent density of the artificial agglomerate stone articlemight be measured according to EN 14617-1:2013-08

The artificial agglomerate stone material may be in the form of a block,slab, tile, sheets, board or plate.

The artificial agglomerate stone material might be used for constructionor decoration, for manufacturing counters, kitchen countertops, sinks,shower trays, walls or floor coverings, stairs or similar.

The invention is also concerned with a process for preparing theartificial agglomerate stone material of the invention, comprising:

-   -   a) mixing a hardenable binder and the inorganic fillers        comprising the synthetic silicate granules of the invention,    -   b) vacuum vibrocompacting the unhardened mixture obtained in a),        and    -   c) hardening the compacted mixture obtained in b).

In an embodiment, vacuum vibrocompacting the unhardened mixture obtainedin a) is performed in a mold or a supporting sheet.

For the manufacture of the artificial agglomerate article, a hardenablebinder, such as a liquid organic resin, is mixed with the syntheticsilicate granules, and with any optional inorganic fillers differentthan the synthetic silicate granules forming an (unhardened) agglomeratemixture. The amount of synthetic silicate granules is preferably 1-70wt. %, or 1-50 wt. %, or 1-30 wt. % of the weight of the agglomeratemixture. The sum of the weights of the synthetic silicate granules andthe optional inorganic fillers different than the synthetic silicategranules is preferably at least 70 wt. %, or at least 80 wt. %, or atleast 85 wt. % of the weight of the agglomerate mixture. Preferably, theamount of hardenable binder in the agglomerate mixture ranges from 5-30wt. %, or from 5-15 wt. %.

In preferred embodiments, the synthetic silicate granules are producedby sintering a mixture according to previous embodiments, comprisingkaolin and a flux.

The mixing can be achieved, for example, by stirring with the use ofconventional mixers, in a manner known in the art. The hardenable bindermight be an organic resin, which once hardened, serves to achievecohesion and adherence between the inorganic fillers in the producedarticle. The organic resins are preferably thermosetting, liquid and canbe selected, for example, from the group made up of unsaturatedpolyester resins, methacrylate-based resins, vinyl resins and epoxyresins. These resins are preferably reactive and harden in a curing orcross-linking reaction. Additionally, additives can be included in thismixing step, selected from pigments, curing catalysts, curingaccelerators, UV stabilizers, or mixtures thereof.

The optional inorganic fillers different than the synthetic silicategranules might be selected from stone, stone-like or ceramic materials,e.g. in granule form. These fillers may be incorporated to theagglomerate mixture with different particle sizes and can be obtainedfrom the crushing and/or grinding of natural or artificial materials.These inorganic fillers can be sourced, for example, from specializedcompanies, which commercialize them already dry and classified accordingto their particle size.

Artificial agglomerate stone materials with a low crystalline silicacontent are preferred. Therefore, it is preferred that all, or at least95 wt. %, or at least 90 wt. % or at least 80 wt. %, of the otherinorganic fillers different from the synthetic silicate granules of theinvention have a low crystalline silica content, preferably acrystalline silica (quartz, cristobalite or other crystallinepolymorphs) content of 0-15 wt. %, or 0-10 wt. %, or 0-7 wt. % relativeto the weight of said other inorganic fillers. Preferably, at least 80%,more preferably at least 90 wt. %, of the other inorganic fillersdifferent from the synthetic silicate granules have a crystalline silicacontent of 0-7 wt. % relative to the weight of said other inorganicfillers.

In particularly preferred embodiments, the artificial agglomerate stonematerial or article comprises 0-5 wt. %, or 0-1 wt. %, relative to theweight of the agglomerate stone material or article, of inorganicfillers different than the synthetic silicate granules, with acrystalline silica (quartz, cristobalite or other crystallinepolymorphs) content of >7 wt. %, or >10 wt. %, or >15 wt. % relative tothe weight of said inorganic fillers.

It is preferred that the artificial agglomerate stone material comprisesfrom 0-5 wt. % relative to the weight of the material, of inorganicfillers (i.e. the sum of the weights of the synthetic silicate granulesand of the inorganic fillers different from the synthetic silicategranules of the invention) with a content of crystalline silica of15-100 wt. % relative to the weight of the inorganic fillers.

Preferably, the crystalline silica content of the artificial agglomeratestone material is wt. %, more preferably wt. %, wt. % relative to theweight of the material. The crystalline silica content of the artificialagglomerate stone material may be 0-15 wt. %, more preferably 0-10 wt.%, or 0-5 wt. %, relative to the weight of the material.

The inorganic fillers different than the synthetic silicate granules arepreferably selected from feldspar granules, recycled silicate glassgranules, silicate frit granules, ceramic granules, or mixtures thereof.It needs to be understood that the inorganic fillers, i.e. granules,different than the synthetic silicate granules have a composition ofoxides different to the composition of the synthetic silicate granulesof the invention here.

The agglomerate mixture may comprise other typical additives, such ascolorants or pigments, accelerators or catalyzers for the curing orhardening of the resin (e.g. free radical initiators), promoters for theadhesion between the filler and the resin (e.g. silanes). These types ofadditives and the proportion used thereof are known in the state of theart. Preferably, these additives may be present in the agglomeratemixture in an amount of 0.01-5.00 wt. %, based on the weight of themixture.

The (unhardened) agglomerate mixture may be then transported to adistributor device. Distributors suitable are known, such as those usedfor the distribution of the (unhardened) agglomerate mixtures in themanufacture of quartz agglomerate surfaces. This distributor device ispreferably movable along the length of a temporary mold or supportingsheet and preferably consists of a feeding hopper that receives themixture in the top opening thereof and a conveyor belt positioned belowthe bottom outlet opening of the hopper, which collects or extracts themixture from the hopper and deposits it into the mold or supportingsheet. Other distributor devices are possible within the general conceptof the invention.

The (unhardened) agglomerate mixture having been distributed in the moldor supporting sheet is preferably covered with a protective sheet on itstop surface and subjected to vacuum vibrocompaction. For this, in anexample, the mixture is transported inside a compaction area of a press,wherein it is inserted in a sealable chamber. Then, the chamber issealed, and vacuum is created with appropriate gas evacuation pumps.Once the desired vacuum level has been reached (e.g. 5-40 mbar), the ramof the press exerts a compaction pressure simultaneously with theapplication of vertical vibration of the piston (e.g. oscillating at2.000-4.000 Hz). During the vacuum vibrocompaction, the air entrapped inthe agglomerate mixture is substantially evacuated.

The compacted mixture then goes to a hardening or curing stage. In thisstage, depending on the type of resin, as well as the use or not of anysuitable catalysts or accelerants, the mixture is suitably subjected tothe effect of temperature in a curing oven, suitably heated at atemperature between 80-120° C., with residence times in the ovengenerally varying from 20 to 60 minutes. After curing, the hardenedcompacted mixture is cooled down to a temperature equal to or less than40° C.

After hardening, the artificial agglomerate article obtained, which canbe shaped as blocks, slabs, boards or plates, can be cut and/orcalibrated to the desired final dimensions, and may be finished(polished, honed, etc.) on one or both of its larger surfaces, dependingon the intended application.

It should be understood that the scope of the present disclosureincludes all the possible combinations of embodiments disclosed herein.

Examples

Definitions and Testing Methods:

XRF: Oxide analysis of the granules might be conducted by X-RayFluorescence in a commercial XRF spectrometer. For example, a disc ofabout 1 g of a sample is mixed with lithium tetraborate and calcined inair atmosphere at a temperature 1.050° C. for 25 minutes prior toanalysis in the spectrometer. The results are reported as relativeweight percentage of oxides (SiO₂, Al₂O₃, etc.), together with theweight ‘lost on ignition’ during calcination (evaporation/desorption ofvolatiles, decomposition of organic matter). The spectrometer ispreviously calibrated with multipoint calibration curves of knownconcentration of standards.

XRD: As way of example, the identification and quantification ofcrystalline phases in the granules can be done by powder X-RayDiffraction (XRD) using MoKai radiation (0.7093 Å) with a commercialequipment (e.g. Bruker D8 Advance) at 2°-35° for 4 hours. Once the X-raydiffraction data is obtained, it is analyzed using the Rietveld methodfor quantification. The content of crystalline silica phases iscalculated as weight percentage of the sample analyzed.

Granulometry: The particle size, also called particle diameterdistribution, of the granules can be measured by known screeningseparation using sieves of different mesh size. For synthetic silicategranules with a particle size <200 micrometers, the particle sizedistribution can be measured by laser diffraction with a commercialequipment (e.g. Malvern Panalytical Mastersizer 3000 provided with aHydro cell). For the measurement, the granule sample might be dispersedin demineralized water assisted by an ultrasound probe. The laserdiffractomer provides particle distribution curves (volume of particlesvs. particle size) and the D10, D50 and D90 statistical values of theparticle population (particle size values where 10%, 50% or 90% of thesample particle population lies below this value, respectively).

Colorimetry/transparency: Colorimetry and transparency of the granulesin polymerized matrix can be measured from disks prepared by mixing 50 gof the granules with 50 g of a commercial unsaturated polyester resincatalyzed with 0.75 g of organic MEKP peroxide and 0.12 g of cobaltoctoate (6% cobalt). After homogenization, the mixture is poured to analuminum mold up to a thickness of 5 mm. The mixture is then hardened at70° for 20 minutes and allowed to reach room temperature afterwards for30-40 minutes. The aluminum mold is then removed before the colorimetryand transparency of the obtained disk is measured. The colorimetry maybe measured in a commercial spectrophotometer (e.g. Konica MinoltaCM-3600d) and expressed in values of L* a* b* coordinates (CIELAB colorspace), where L* is lightness from black (0) to white (100), a* fromgreen (−) to red (+) and b* from blue (−) to yellow (+). Transparencymay be measured in a commercial transparency analyzer (e.g. from SensureSRL) capable of measuring the ratio of white light transmitted throughthe disk.

Resin absorption: The absorption of resin is measured by addingcommercial liquid unsaturated polyester resin dropwise from a burette to5.0 g of a sample of the granules placed on a glass plate. The mass ofgranules and oil is rubbed and mixed thoroughly with a stainless-steelspatula. Drops of resin are added until the mass reaches the consistencyof a stiff, putty-like paste that does not break or separate, with a dryappearance, and which remains adhered to the spatula (called the“pick-up” point). In that moment, the amount of resin used to reach thepick-up point is recorded and the resin absorption calculated as % inrelation to the initial weight of the sample.

In an Example 1, a mixture was prepared under efficient stirring bycontacting 90 weight parts of commercial high purity washed kaolin withan Al₂O₃ content of >30 wt. % and a Fe₂O₃ content of <0.7 wt. %, with anaverage particle size <30 micrometers, and 10 weight parts of highlypure floated sodium feldspar with a NaO content of >10 wt. % and Fe₂O₃of <0.1 wt. %, with an average particle size <100 micrometers.

The mixture obtained was then compacted in an axial press with apressure of 420 kg_(F)/cm².

After compaction, the mixture was located into a crucible and entered toa muffle-type furnace which was then set to 1400° C. The mixture wasleft inside the furnace for 12 minutes at the maximum temperature, inwhich period it underwent sintering. Afterwards, the sintered mixturewas left to slowly cool-down to room temperature. The produced syntheticsilicate granules were obtained by grinding and/or micronizing thesintered mixture. The granules were then classified by sieving accordingto fractions of different particle size ranges.

In an Example 2, the same experimental protocol was followed as forExample 1, but adding 85 weight parts of kaolin and 15 weight parts offeldspar.

Table 1 depicts the average composition of the synthetic silicategranules obtained in these Examples 1-2 measured by XRF (indicatedvalues correspond to wt % based on the weight of the granules):

TABLE 1 Other L.O.I. SiO₂ Al₂O₃ Na₂O CaO K₂O MgO Fe₂O₃ TiO₂ oxides Ex. 10.05 57.61 39.11 1.31 0.32 0.66 0.18 0.47 0.15 0.14 wt. % Ex. 2 0.1758.78 37.3 1.94 0.37 0.53 0.19 0.41 0.15 0.16 wt. %

The hardness of the synthetic silicate granules of Examples 1-2 is 6 inthe Mohs scale. The average content of crystalline silica in thesynthetic silicate granules of Example 1, as measured by DRX with theRietveld quantification method, is 3.0 wt. % in the form of quartz and5.1 wt. % cristobalite. For Example 2 it is measured 3.1 wt. % quartzand 4.9 wt. % cristobalite. The average content of the crystalline phasemullite is 45 wt. % for Example 1 and 43 wt. % for Example 2.

The colorimetry and transparency of the synthetic silicate granuleshaving different granulometry obtained according to Examples 1-2, in apolymerized resin matrix, is shown in Table 2, together with thecolorimetry and transparency of quartz and cristobalite granules ofsimilar granulometry for comparison. The absorption of resin of themicronized synthetic silicate granules obtained according to Examples1-2 is also presented in Table 2, together with the absorption valuesobtained for micronized quartz and cristobalite granules of similarparticle size.

TABLE 2 Transparency Resin Colorimetry % light absorption L* a* b*transmitted wt. % Granules of Example 1 86.4 0.5 6.6 7.3 — Particle sizerange 0.1-0.4 mm Granules of Example 2 84.4 −0.4 4.7 7.3 — Particle sizerange 0.1-0.4 mm Cristobalite, particle size 87.6 0.9 2.4 16.1 — 0.1-0.4mm Quartz, particle size 84.4 0.4 4.8 20.5 — 0.1-0.4 mm Micronizedgranules of Ex. 1 77.9 −0.1 3.3 7.4 27 D90 = 35.0 micrometers Micronizedgranules of Ex. 2 78.4 0.0 2.3 7.5 24 D90 = 35.0 micrometers Micronizedcristobalite, 81.9 0.7 1.2 9.0 34 D90 = 22.0 micrometers Micronizedquartz, 47.5 2.2 4.6 11.0 25 D90 = 27.1 micrometers

The quartz and cristobalite granules included in Table 2 as referenceare commercial materials currently being used in the manufacture ofartificial agglomerate quartz stone articles.

As can be seen from the results shown herein, the synthetic silicategranules can be produced from readily available raw materials and at acompetitive cost. The granules have furthermore a combination ofcharacteristic which make them suitable as toxicologically safermaterial for replacing quartz or cristobalite granules in themanufacture of artificial agglomerate stone articles, without having tochange the materials and processes normally used for the manufacture ofquartz agglomerate surfaces. These features are:

-   -   Can be obtained by thermal transformation at temperatures <1450°        C.    -   Have high hardness and good chemical/mechanical resistance.    -   Show low content of crystalline silica and/or other        toxicologically problematic substances (such as lead, cadmium,        etc.)    -   Present high lightness (whiteness), similar to quartz or        cristobalite. The color tonalities of the synthetic silicate        granules show slightly deviations from the L* a* and b* values        obtained for either quartz or cristobalite. The slightly higher        b* values on the synthetic silicate granules of Example 1        indicate that in that case, when mixed with resin, the synthetic        silicate granules will turn the polymerized mixtures slightly        more yellow than in the cases of quartz or cristobalite.        However, this difference is low enough to be possible the        adjustment with pigments.    -   The synthetic silicate granules result in a lower transparency        than the quartz and cristobalite granules. This difference is        less pronounced when the granules have smaller particles, i.e.        when they are micronized. Taking this result into consideration,        the synthetic silicate granules may be used in the manufacture        of artificial agglomerate stone articles which are mostly opaque        and do not require this granule transparency. On the other hand,        the transparency requirement for the manufacture of artificial        agglomerate stone articles is of lower relevance when the        granules are used micronized.    -   The absorption of resin of the micronized synthetic silicate        granules is not higher than the absorption of either quartz or        cristobalite micronized granules.

The synthetic silicate granules obtained in Example 1 were used for themanufacture of artificial agglomerate stone slabs in an industrialsetting, in standard lines for the production of commercial quartzagglomerate surfaces.

Micronized synthetic silicate granules with a D90 of 35.0 micrometerswere used to replace partially or fully the micronized cristobalitenormally used. On the other hand, the synthetic silicate granules ofExample 1 with a particle size distribution 0.1-0.4 mm were used toreplace partially or fully the quartz granules of similar granulometrynormally used.

In all the cases, the slabs could be manufactured without problems orimportant changes in the current production process, only with a slightadjustment of the concentration of the pigments used. The slabscomprising the synthetic silicate granules showed similarcharacteristics regarding resistance to abrasion, scratch, staining orchemical attacks as the slabs produced with cristobalite and quartz.However, the slabs with the granules of the invention contained a lowercontent of crystalline silica, which resulted in lower emission ofrespirable crystalline silica when the slabs were cut, gauged and/orpolished.

The invention claimed is:
 1. Synthetic silicate granules comprising:52.5-59.8 wt % of SiO₂, 33.5-41.1 wt % of Al₂O₃, and 0.3-3.1 wt % ofNa₂O, based on the weight of the synthetic silicate granules, whereinthe granules comprise a crystalline phase in a range 5-80 wt. %, basedon the weight of the synthetic silicate granules.
 2. Synthetic silicategranules according to claim 1, wherein the granules comprise Fe₂O₃ in arange 0.00-1.00 wt. % based on the weight of the synthetic silicategranules.
 3. Synthetic silicate granules according to claim 1, whereinthe granules comprise crystalline silica in a range 0-15 wt. % based onthe weight of the synthetic silicate granules.
 4. Synthetic silicategranules according to claim 1, wherein the granules comprise crystallinemullite in a range 20-60 wt. %, based on the weight of the syntheticsilicate granules.
 5. Synthetic silicate granules according to claim 1,wherein: the synthetic silicate granules comprise: 56.90-59.80 wt. % ofSiO₂, 33.50-41.10 wt. % of Al₂O₃, and 0.90-3.10 wt. % of Na₂O, based onthe weight of the synthetic silicate granules.
 6. Synthetic silicategranules according to claim 1, wherein the sum of the amount of SiO₂,Al₂O₃and Na₂O in the synthetic silicate granules is 86.30-99.80 wt. %based on the weight of the synthetic silicate granules.
 7. Syntheticsilicate granules according to claim 1, wherein the synthetic silicategranules further comprise: 0.3-6.9 wt. % of CaO, and/or 0.3-3.1 wt % ofMgO, and/or 0.0-0.5 wt % of TiO₂, and/or 0.0-2.0 wt. % of K₂O, based onthe weight of the synthetic silicate granules.
 8. Synthetic silicategranules according to claim 1, wherein the sum of the amount of SiO₂,Al₂O₃and Na₂O in the synthetic silicate granules is 95.00-99.50 wt. %based on the weight of the synthetic silicate granules.
 9. Artificialagglomerate stone material comprising inorganic fillers and a hardenedbinder, wherein the inorganic fillers comprise synthetic silicategranules as defined in claim
 1. 10. Artificial agglomerate stonematerial according to claim 9, wherein: the inorganic fillers furthercomprise inorganic fillers different than the synthetic silicategranules selected from feldspar granules, recycled silicate glassgranules, silicate frit granules, ceramic granules, and mixture thereof.11. Artificial agglomerate stone material according to claim 9,comprising from 0-5 wt. % relative to the weight of the material, ofinorganic fillers with a content of crystalline silica of 15-100 wt. %relative to the weight of the inorganic fillers.
 12. Artificialagglomerate stone material according to claim 9, wherein: the amount ofsynthetic silicate granules is from 1 to 70 wt. % based on the weight ofthe artificial agglomerate stone material.
 13. Artificial agglomeratestone material according to claim 9, characterized in that it has anapparent density from 2000-2600 kg/m³.
 14. Artificial agglomerate stonematerial according to claim 9, wherein the amount of inorganic fillersis at least 70 wt. % based on the weight of the artificial agglomeratestone material.
 15. Artificial agglomerate stone material according toclaim 9, wherein the amount of inorganic fillers is at least 80 wt. %based on the weight of the artificial agglomerate stone material. 16.Artificial agglomerate stone material according to claim 9, wherein theamount of synthetic silicate granules is from 1 to 50 wt. % based on theweight of the artificial agglomerate stone material.
 17. Artificialagglomerate stone material according to claim 9, wherein the amount ofthe synthetic silicate granules with a particle size ≤0.063 mm is 10-40wt. % in relation to the weight of the artificial agglomerate stonematerial.
 18. Artificial agglomerate stone material according to claim13, characterized in that it is produced in a method including a vacuumvibrocompaction step.
 19. A process for preparing the artificialagglomerate stone material as defined in claim 10, comprising: a) mixinga hardenable binder and inorganic fillers comprising synthetic silicategranules comprising: 52.5-59.8 wt % of SiO₂, 33.5-41.1 wt % of Al₂O₃,and 0.3-3.1 wt % of Na₂O, based on the weight of the synthetic silicategranules, wherein the granules comprise a crystalline phase in a range5-80 wt. %, based on the weight of the synthetic silicate granules, b)vacuum vibrocompacting the unhardened mixture obtained in a), and c)hardening the compacted mixture obtained in b).